This invention relates in general to helical scan magnetic tape record/reproduce apparatus and, in particular, this invention relates to a technique for changing the helix angle of magnetic tape relative to the rotating heads of the rotary head scanner.
Helical scan magnetic tape record/reproduce apparatus are widely used to record analog or digital inFormation on slant tracks on magnetic tape. Generally, the magnetic tape is contained in a two-reel cartridge or cassette. The magnetic tape recorder includes a tape transport assembly (including stationary and rotating rollers, tension devices, and capstans) to move the tape past the rotating heads oF a rotary head scanner, from supply and to take-up reels of the tape cassette. The rotary head scanner includes upper and lower stationary drums and a head wheel rotatably mounted in a slot between the stationary drums. The head wheel includes one or more magnetic heads mounted on the periphery of the head wheel. The lower drum of the rotary head scanner, typically, has a helical shelf which guides the lower edge of the magnetic tape as it is transported past the rotary head scanner. The tape is oriented relative to the head paths by a pair oF tilted guides at the exit and entry to the rotary head scanner. The guides establish the helix angle oF the path of the tape as it travels around the rotary head scanner.
An elaborate and complicated mechanism is commonly employed for threading the magnetic tape from the cassette along its transport path past the head scanner. Typically, the threading mechanism includes rollers and pulleys mounted on slides and linkages to extract the tape from the cassette and to wrap it around the rotary head scanner. Such mechanical threading mechanisms are complex, noisy and subject to breakdown.
In commonly assigned U.S. Pat. Nos. 4,763,210, issued Aug. 9, 1988, entitled HELICAL SCAN TAPE GUIDE APPARATUS and 4,772,969, issued Sept. 20, 1988, entitled "EXPANDABLE LOOP" METHOD AND APPARATUS FOR AUTOMATICALLY THREADING A WEB MATERIAL, there is disclosed a magnetic tape threading and guiding apparatus including a pair oF concave air-bearing guides disposed immediately adjacent and tangential to opposing sides of a circumferential surface of a rotary head scanner to cooperatively form a path for guiding magnetic tape past the head scanner at a predetermined helix angle. A vacuum source serves to evacuate air from the vicinity oF each air-bearing surFace to create an air pressure gradient, the effect of which is to cause magnetic tape to ride on a thin film of air interposed between the tape and each air-bearing guide during transport oF the tape from a supply reel to a take-up reel. Such a tape threading and guide apparatus, offers several advantages over tape-threading and guide mechanisms of the prior art. For example, since tape threading is fully pneumatic, the threading apparatus is simplified and less expensive. Moreover, tape is threaded and guided in a more gentle and less damaging manner than mechanical devices, thus providing increased efficiency and minimized tape damage.
Ideally, in a helical scan magnetic record/reproduce apparatus, the information recorded in slant tracks on magnetic tape is reproduced by magnetic heads which scan the slant tracks in proper alignment with and directly over the tracks so that an optimal signal is reproduced. However, if a magnetic tape is reproduced at a tape speed different than the speed at which the tape was recorded, the magnetic heads will sweep across the magnetic tape at an angle different than the angle of the slant tracks on the tape. Thus, the reproduce head scans not only the desired track, but also adjacent tracks, causing signal degradation and noise in the reproduced signal. For example, if the reproduce magnetic tape apparatus is operated in a still mode or slow or fast, forward or rewind scan modes, the reproduced signal will be substantially degraded from the recorded signal. In many applications, this is undesirable, for example, when reproducing images at a slow scan rate in order to analyze the motion in a scene recorded at a fast frame rate.
One solution to this problem, is to change the helix angle of wrap of the magnetic tape about the rotary head scanner in order to align the recorded tracks on the magnetic tape with the path oF the rotating magnetic heads. U.S. Pat. No. 3,697,676, issued Oct. 10, 1972, entitled HEAD-TO-TAPE APPARATUS AND METHOD, discloses several techniques for changing the angle formed by the track on magnetic tape and the path of a rotating magnetic head. As shown in FIG. 2 of the latter patent, one technique is to vary the position of entry and exit guides to the rotary head scanner in order to change the helix angle of the path of magnetic tape about the head scanner, so that the path of the reproduce heads are in alignment with the tracks on magnetic tape. Another technique disclosed in FIG. 5 of the latter patent, utilizes a mechanism for changing the angle of rotation of the head wheel of the rotary head scanner relative to the tracks on magnetic tape in order to align the reproduce heads with the recorded tracks. U.S. Pat. No. 4,703,370, issued Oct. 27, 1987, entitled HELICAL SCAN TYPE MAGNETIC TAPE REPRODUCING APPARATUS WITH VARIABLE HEAD-DRUM INCLINATION CAPABILITY, also discloses a mechanism for varying the angle of rotation of the magnetic heads relative to the magnetic tape. Other patents showing adjustment of the helix angle of wrap of magnetic tape about a rotary head scanner, include U.S. Pat. No. 4,709,280, issued Nov. 24, 1987, entitled APPARATUS FOR VARYING THE PATH OF MAGNETIC TAPE PASSING OVER A ROTARY HEAD; U.S. Pat. No. 3,418,424, issued Dec. 24, 1968, entitled MAGNETIC RECORDING AND REPRODUCTION OF TELEVISION SIGNALS; and IBM Technical Disclosure Bulletin, Vol. 18, No. 12, pages 3931-3934, dated May 1976, entitled DISTORTION FREE TAPE STEERING MECHANISM. Each of the techniques, disclosed in these latter documents, use an adjustable tape guide for varying the helix angle of wrap of magnetic tape about a rotary head scanner.
The known techniques for varying the helix angle of wrap of magnetic tape about a rotary head scanner (as exemplified by the patents and publications referred to above) are disadvantageous because of the complex mechanical arrangements for guiding the tape during its transport past the rotary head scanner and because of the potential damage to the tape in varying tape guide elements.